Quadrature detectors are conventionally used as a means of detecting the phase changes of a high-frequency signal. An example of this kind of detector is shown in FIG. 2 where an input signal S1 is multiplied by the double-balanced mixers 21 and 22 using local signals S21 and S22, respectively. The signals S21 and S22 are generated by a local oscillator 23, and the phase of local signal S21 is shifted by a ninety degree phase-shifter 24, so that the phases of signals S21 and S22 are ninety degrees apart. The outputs S23 and S24 of mixers 21 and 22 are applied to low-pass filters 25 and 26, respectively, which remove the high frequency components of signals S23 and S24 and produce baseband signals S25 and S26 with phases which are orthogonal to each other. Based on a sampling pulse P, analog-to-digital (A/D) converters 27 and 28 convert S25 and S26, respectively, to digital signals which are then outputted as detection results or as I and Q signals.
The above process can be described by the following mathematical expressions. Let the input signal S1 be EQU x=2.multidot.A cos (.omega..sub.c t+.phi.) (1)
and let the local signals S21 and S22 be respectively EQU y.sub.21 =cos (.omega..sub.c t), EQU y.sub.22 =-sin (.omega..sub.c t), (2)
then, the outputs S23 and S24 of mixers 21 and 22 are respectively given by EQU Z.sub.23 =x.multidot.y.sub.21 =A cos (2.omega..sub.c t+.phi.)+A cos (.phi.)(3) EQU Z.sub.24 =x.multidot.y.sub.22 =-A sin (2.omega..sub.c t+.phi.)+A sin (.phi.). (4)
where,
.omega..sub.c : the angular frequency of the carrier of an input signal S1(radian/sec) PA1 t: time (sec) PA1 .phi.: the phase of input signal S1(radian).
The values of the I and Q signals whose high frequency components are removed by the low-pass filters 25 and 26 are respectively EQU I=A cos (.phi.), Q=A sin (.phi.). (5)
The phase .phi. can be derived from the I and Q signals according to the relationship, EQU .phi.=tan.sup.-1 (Q/I). (6)
The foregoing describes the principle behind the operation of a conventional quadrature detector.
The conventional quadrature detector described above has several problems as follows;
(1) It is very difficult to accurately implement the ninety degree phase-shifter (24 in FIG. 2). An error with the phase-shifter directly results in an error in the phase detection.
(2) Since it is difficult to accurately balance the two mixers 21 and 22, a DC off-set inevitably appears in the baseband signal. This also results in an error in the phase detection.
(3) An amplitude deviation appears where an unbalance occurs between the mixers 21, 22 and the low pass filters 25 and 26. This also causes an error in the phase detection.
To reduce these errors, a considerable amount of adjusting is required. Even if these adjustments are successful, the errors caused by fluctuations in the characteristic values of the devices as caused by temperature changes or aging cannot be prevented. In addition, the integration of analog circuits such as mixers, a ninety degree phase-shifter and low pass filters is difficult, making reductions in the size of the detector and power consumption difficult.